Certain clinical and morphologic features are described in 23 patients in whom the heart at necropsy weighed at least 1,000 g (mean 1,106). The heart weight to body weight ratio ranged from 1.2 to 2.7 (normal 0.40). The 23 patients were derived from examination of the hearts of 7,671 patients with various cardiovascular disorders over a 25-year period. The massive cardiomegaly was the result of aortic regurgitation in 14 patients (61%): isolated in 8, associated with mitral regurgitation in 4, and with ventricular septal defect in 2. Three others (13%) had combined aortic valve stenosis and aortic regurgitation and 1 patient (4%) had mitral stenosis and regurgitation and mild aortic stenosis. Four patients (17%) had hypertrophic cardiomyopathy, and 1 patient (4%) had ventricular septal defect with mitral stenosis. They were 20 to 64 years old (mean 442) and 21 (91%) were men. Four patients at necropsy had 1 or more major coronary arteries narrowed more than 75% in cross-sectional area by atherosclerotic plaques, and only 4 patients had grossly visible left ventricular (LV) scars, 2 of whom had insignificant coronary narrowing. Examination of electrocardiograms in 17 of the 23 patients disclosed that Sokolow-Lyon criteria for LV hypertrophy was achieved in only 12 patients (71%) and Romhilt-Holt QRS voltage criteria faired even worse. Total 12-lead QRS voltage was more than 175 mm (10 mm = 1mV) in 16 patients (94%) and it was more than 250 mm in 13 patients (76%). Total 12-lead QRS voltage in 17 patients ranged from 140 to 601 mm (mean 323). Measurement of the sum in diagnosing LV hypertrophy by electrocardiogram.